In an image forming apparatus for forming an image using an electrophotographic process, the image characteristics such as image density are affected by a chronological change of the materials such as a photoreceptor and developer, a change of an ambience, namely, a change in the temperature and humidity, and so on. To solve this problem, attempts have been made to provide image quality control for stabilizing the image quality by corrections conforming to these fluctuating factors.
In Japanese Unexamined Patent Application Publication No. H5-323743, a reference electrostatic latent image called a “patch image” is formed on a photoreceptor by charging and exposure, and the potential of the reference electrostatic latent image is detected and stored. When an image is formed, an electrostatic pattern is formed under the same conditions as those for forming a reference electrostatic latent image, and the image forming conditions are set based on the result of detecting the potential of this electrostatic pattern.
Other documents than Japanese Unexamined Patent Application Publication No. H5-323743 also disclose the aforementioned image quality control technique wherein a reference image is formed, and the potential and density of the formed reference image are detected, whereby the image forming conditions are controlled based on the result of detection.
An effective control method for maintaining a certain image quality level is provided by the image quality control technique disclosed in the Japanese Unexamined Patent Application Publication No. H5-323743 wherein a reference image is formed, and the potential and density of the reference image are detected, whereby the image forming conditions are controlled based on the result of detection.
In this method, however, when the image forming characteristics such as development characteristics have been subjected to a drastic chronological change in a predetermined direction, insufficient correction from the target image quality will be disabled immediately after the change has been corrected. Thus, it has been found out that image quality problems such as density occur.
To be more specific, when there is a drastic change in the environment as exemplified by temperature or humidity or in the page coverage rate of the image, correction fails to catch up with the needs, with the result that a fluctuation in image quality occurs. This problem raises a big concern in printing especially with respect to a plate making phase. Such problems cannot be solved by the conventional arts including the image quality control disclosed in Japanese Unexamined Patent Application Publication No. H5-323743.
The object of the present invention is to provide an image forming apparatus capable of suppressing a change in image quality caused by the chronological change in image forming characteristics, thereby ensuring stable formation of a high-quality image.